Need help with my writing homework on Bacteriology. Write a 500 word paper answering; Bacteriology Case Study A. Calculate the sensitivity, specifi positive predictive value and negative predictive value for the Toxin A and B EIA, the GDH IHC and the PCR for toxin B gene using the cytotoxin assay as the Gold Standard.


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Positive Predictive Value

Negative Predictive Value

toxin A and B










PCH (toxin)





The formulae utilised in calculating these values are as shown below. All values are expressed as percentages for easy interpretation of the data.

Sensitivity = a&nbsp./&nbsp.(a+b)

Specificity = d&nbsp./&nbsp.(c+d)

Positive predictive value = a&nbsp./&nbsp.(a+c)

Negative predictive value = d&nbsp./&nbsp.(b+d)

For the purposes of these equation the values are as follows

A=true positive

B= false negative

C= false positive

D= true negative

B. Assuming that it is not feasible to use the cytotoxin assay in a routine diagnostic laboratory service, discuss which of the other three tests you would recommend as useful and how they might be used (e.g. is one of the assays suitable on its own? If not how might they be used in combination?)

Numerous factors are considered when selecting the testing method utilised in identification of C. difficile microorganisms. The selected methods must be affordable for the organisation undertaking the testing. Since the different methods require different testing durations, the number of samples and the time results expected also determine the testing methods to be utilised (DuPont 2011). To ensure that all these factors have been met, optional methods can be utilised either singly or by combining two methods. The best option for testing would be combining Glutamate dehydrogenase (GDH) assay with enzyme immune-assay (EIA). The combination of these methods would enhance the limitations experienced by each method and increase the reliability of results.

Samples would be prepared and each sample tested using both methods, and results recorded separately. A comparison of the results would then be undertaken to establish the various attributes being analysed by the results. The GDH method has been identified as having high sensitivity, but very poor specificity. The method can, therefore, accurately rule out the presence of clostridium difficile, but cannot ascertain the presence of the microbes (Goldenberg et al. 2010). The method rarely produces a negative for samples labelled true-positive using the EIA testing method. In most testing processes, GDH produces 100% specificity result because of the lack of false positive result, from the testing.

The combination of these methods would enhance findings since GDH produces high sensitivity, and EIA produces relatively high specificity. Combining these methods, therefore, presents researchers with a capacity to have reliable results from the laboratory testing of clostridium difficile. GDH accurately tests the presence of enzymes produced by these microorganisms, but cannot ascertain the presence of C. difficile since similar enzymes are produced by other bacteria (Eastwood et al. 2009). The method can, therefore, rule out the presence of the microbial strain with a negative result, but a positive result cannot ascertain the presence of the microbial strain. Since the methods do not produce 100% sensitivity, the increased number of samples when undertaking a combined testing increases the accuracy of results.


DuPont, H.L., 2011. Approach to the patient with suspected enteric infection. In L. Goldman & A. I. Schafer, eds. Cecil Medicine. Philadelphia: Elsivier.

Eastwood, K. et al., 2009. Comparison of nine commercially available Clostridium difficile toxin detection assays, a real-time PCR assay for Clostridium difficile tcdB, and a glutamate dehydrogenase detection assay to cytotoxin testing and cytotoxigenic culture methods. Journal of Clinical Microbiology, 47, pp.3211–3217.

Goldenberg, S.D., Cliff, P.R. & French, G.L., 2010. Glutamate Dehydrogenase for Laboratory Diagnosis of Clostridium difficile Infection. Journal of Clinical Microbiology, 48 (8), pp.3050–3051.

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